Quantum Entanglement

Spooky Behaviour

This section describes some of the strange behaviours seen in experiments.

Double-slit Experiments

Figure 6.1 illustrates what happens when a source of light shines
through two tiny slits onto a screen. The detector screen illuminates a
wave-like pattern caused by light interfering with itself. This is how
waves are expected to behave.

Figure 6.1.Experiment 1 - Light Waves.

Since photons are also particles, we can transmit them one at a time.
Figure 6.2 shows the result of many solitary photons being fired at
the detector.

Figure 6.2.Experiment 2 - Light Particles.

The interference pattern still appears; but if photons are fired
alone, then with what do they interfere? Quantum theory tells us that each
photon interferes with itself. If true, then it implies that we cannot know
through which slit the photon travels; the photon seems to have travelled
both slits simultaneously!

Trying to detect which slit the photons travel puts this quantum weirdness in
the spotlight, so to speak.

For example, we can polarize the light before it goes through the slits.
Like rippling a rope through a picket fence to permit only vertical waves
(see Figure 4.4), polarizing allows us to limit the type of light waves
that make it through the slits to the detector.

When we put a polarizing filter around one of the two slits, the
interference pattern disappears. The result is shown in Figure 6.3.

Figure 6.3.Experiment 3 - Polarized Light Waves.

Whenever we can detect, or deduce, through which slit a photon has travelled,
the interference pattern instantly disappears. An interference pattern
only appears when the photon's path is unknown.

It gets weirder.

Even if we examine the photon's trail after it passes the double-slits (but
before it reaches the detector), the interference pattern disappears. And it
disappears regardless of whether the examination uses a direct or indirect
measurement of the photon.

But what if we used two photons that are inextricably linked (through
entanglement), to perform the experiment?

Entanglement Experiments

I was born not knowing and have had only a little time to change that here and there.

Richard Feynman, Letter to Armando Garcia.

Parametric Down-conversion

We have already seen how to create entangled photons through a process called
spontaneous parametric down-conversion:

To review, the laser in Figure 6.4 fires a high-energy beam into a special
type of crystal. Every once in a while one of the photons from the beam
will split into two less energetic photons. These two entangled photons will
have opposite polarizations and travel in two different directions, resulting
in two streams of polarized light.

The previous double-slit experiments detected interference patterns by
shining a single light source through two slits (Figure 6.1 and Figure 6.2).
The next experiment uses two streams of entangled photons.

Quantum Eraser

Figure 6.5 shows a Bell-state quantum eraser, named
after John Bell.
It illustrates the application of the following steps:

a laser fires photons into a Beta Barium Borate (BBO) crystal;

the crystal entangles some of the photons; and then

entangled photons travel to two different detectors: A and B.

Placed between the crystal and detector B is a double-slit, like in the
previous experiments. Immediately in front of detector A is a
polarizing filter that can be rotated. Figure 4.5 showed an experiment
using sunglasses to see the effects of rotating a polarizer. Those same
effects apply here.

Figure 6.5.Experiment 4 - Bell-state Quantum Eraser.

The Bell-state quantum eraser has one more feature: each slit is covered by
a substance that filters the polarization of a photon. Consequently, the
left-hand slit will receive photons with a counter-clockwise polarization,
and the right-hand slit will pass photons with a clockwise polarization.

Note: Polarization does not affect interference patterns.

Initially, neither detector shows an interference pattern. Since we control
the polarization of photons passing through the slits and we know the
polarization accepted by each slit, we can deduce which way the photons
travelled (counter-clockwise through the left; clockwise through the right).
Thus no interference patterns are detected.

However, if we rotate the polarizing filter in front of detector A so that the
polarizations of the photons that hit the detector are the same (that is,
we can no longer distinguish between clockwise and counter-clockwise
polarizations), then the interference pattern appears at both detectors!

How do the photons arriving at detector B know that the polarizations have
been "erased" at detector A?

Conclusion

Quantum Theory is continually being challenged and tested; physicists are
finding new ways to explain the world of the tiny. Each passing year
brilliant minds add to, or subtract from, the pool of knowledge about quantum
behaviour.

Unlike the static nature of the web pages presented here, quantum physics
is ever changing. Physicists are confronted with problems that will take
many iterations, many years, to solve. Scores of theories will be presented,
some of them merely tweaking, while others radically alter, our perceptions
of quantum nature.